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Questions loom following Blue Line crash, including who started the runaway train, and why didn't a system set up for such an occurrence automatically stop it? Phil Rogers reports in this NBC 5 Investigates segment. (Published Monday, Sept. 30, 2013)

More than 30 people were injured Monday morning after two trains running on the Chicago Transit Authority’s Blue Line tracks collided, CTA officials confirmed.

The accident happened just before 8 a.m. at Harlem Avenue and Interstate 290 in suburban Forest Park. A westbound train that stopped at the Harlem station was struck by an out-of-service train heading toward the Loop, officials said.

CTA officials said 33 people were transported to nine area hospitals. Forest Park Mayor Anthony Calderone told reporters most of the transported riders complained of neck or back pain.

Police initially treated the area as a crime scene because the out-of-service train may have been stolen, sources said, but the CTA disputed the train was taken without authorization. The National Transportation Safety Board has since taken over the investigation.

"We're exploring everything," an NTSB spokesman said. "We want to establish that the signal system worked, that's the first thing you want to rule out. Then you want to rule out mechanical issues ... then you look at operations."

Robert Kelly, of Amalgamated Transit Union 308, said it's still unclear what happened and how the train got out of the station.

"Both the supervisor in the station at Forest Park and the motorman who was sitting in the station said there was nobody on the train as it went through and collided with the other train," Kelly said.

"This is baffling everybody," he said.

CTA spokesman Brian Steele said there are more questions than answers as staff reviews surveillance video and talks to employees.

"We don't know what the circumstances are that led to this train to begin moving on the path that it did," Steele said. "It shouldn't have done so and the question of why is what we're looking into."

Blue Line service between Forest Park and Austin was suspended after the incident, and shuttle buses were in place.

NBC 5 Investigates has learned that the train began its journey at the CTA's Forest Park yard, where it had been parked and awaiting service since Sept. 23.

By the time it traveled the short distance to the Forest Park station on Monday morning, eyewitnesses said they saw nobody in the motorman's cab.

It's believed to have continued on, unmanned for about half of a mile to the spot where it collided with the other train car.

"That train never should have made it to the Forest Park station. It should have been tripped in the yard," Kelly said.

Someone had to start the train, and even if they bailed out, it had to travel up an incline and pass through three different fail safe systems designed to stop it. The last fail safe is located in the cab itself, designed to alert a motorman of impending danger ahead.

"It's a red indication in the cab, when you are approaching something, another train or object, a broken rail, that goes off," Kelly said. "The motorman has to go brake within seconds. If they don't go to brake, the automatic train control will cut them off, and the train is supposed to come to a stop."

All El trains work from a common key, and sources tell NBC 5 Investigates that there are a lot of train keys out there unaccounted for.

It could explain how the train was started, but not how it was able to travel across so many safeguards designed to stop it without someone aboard.

It's also unclear why no one in the CTA command center saw the disaster coming.

"We have people working in these yars all over the system, and if we have trains that can start moving on their own, our people are in danger out there," Kelly said.

Witnesses said at least one person was taken away on a stretcher, but the CTA tweeted the injuries were "minor."

Loyola University Medical Center confirmed they received four patients from the collision but said their injuries were "not serious."

Westlake Hospital in Melrose Park said they were treating two patients in good condition and West Suburban Medical Center in Oak Park reported they received one patient in good condition.

Martinez Butler, a locomotive engineer for another train company, was standing at the corner when the trains collided.

"There was a train that was stopped at the station. Another train came through and looked like it blew the signal, because I heard beeping," she said. "I'm a locomotive engineer. I know the systems, I know the sounds. When you hear those beepings it's warning you that there is an obstruction in front of you and you need to stop."